Hardenable steel material containing aluminum



Dec. 24, 1968 sus'uMu GODA ETAL 3,418,110

HARDENABLE STEEL MATERIAL CONTAINING ALUMINUM Filed Oct. 31, 1967 & NLWW Mm & a u

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m mm F -IIR KT MX S GMOTO FIGZ ATTORNEYS United States Patent 1 Claim.ci. 75-124 ABSTRACT OF THE DISCLOSURE The present invention is toprovide an alloyed steel having excellent mechanical properties and highhardenability obtained by adding Al in an amount of 0.06 to 0.15 wt.percent to a steel consisting of 0.25 to 0.65 wt. percent C, 0.005 to0.30 wt. percent Si, 0.20 to 2.0 wt. percent Mn, 0.03 to 1.50 Wt.percent Cr, 0.03 to 0.50 wt. percent Mo, 0.005 to 0.30 wt. percent V,0.0005 to 0.003 Wt. percent B and balance being Fe and unavoidableimpurities.

This application is a continuation-in-part of application Ser. No.426,231 filed Jan. 18, 1965, now abandoned.

This invention relates to steel material having a high hardenabilityobtained by largely improving the hardenability of steel by the additionof small amount of aluminum.

It is well lmown that a small amount of aluminum may be added to anordinary carbon steel or magnetic steel with the following purposes: inthe case of an ordinary carbon steel with the purpose or fixingdetrimental elements such as nitrogen by causing them to combine withthe added aluminum and making the crystalline grains fine, and in thecase of a magnetic steel with the purposes of improving the magneticproperties of the steel. The amount of aluminum to be contained in themagnetic steel is published to be usually 0.01 to 0.50 wt. percent. But,in this case it is contemplated to obtain the improvement of themagnetic properties by the collaboration with silicium contained in thesteel in a range of from 2 to 5%.

Besides, heretofore it has been also known to cause a steel to containseveral percents of aluminum in order to improve the heat-resistance,acid-resistance and alkaliresistance of the steel. However, there hasnever been made reference to the effect of improving the hardenabilityby the addition of aluminum.

When carrying out the hardening of a steel material, there may beenumerated several factors which determine a the hardening efficiency.The factor to be mentioned at first is the composition of austenite,that is, the amounts of carbon and other alloying elements contained inthe steel. It is well known that the hardenability of steel may beimproved, if the contents of carbon and other alloying elements aregreat. However, in the case of structural steel and the like it is notdesirable to increase the content of carbon in view of the weldabilityand toughness of the steel. Further, it is economically disadvantageousto add various kinds of alloying elements for the purpose of improvingthe hardenability of steel.

The hardenability of steel is also influenced by the austenitic grainsize. It is said that the hardenability may be improved by enlarging theaustenitic grain size. Further, the amount or the state of distributionof carbide particles in the austenite, or the existance of non-metallicimpurities such as oxide or sulfide in the steel are said to be thefactors exerting influences on the hardenability of the steel.

In general, the hardenability of steel does not come so much in questionwhen heat-treating a steel material of relatively small diameter, but inthe case of heat-treating a massive steel material there occur oftentroubles of the steel material lacking in uniformity in quality or ofthe desired strength or toughness being not obtained on account of theshortage of hardening due to the low hardenability of the steelmaterial.

On the other hand, the high hardenability of the steel material bringthe advantages of being able to easily stabilize the quality of thesteel material at the desired state and of extremely simplifying theheat-treatment of the material.

Heretofore, the most effective method of obtaining a high hardenabilityof steel material, which brings advantages as above mentioned, was saidto be the addition of alloying metals such as manganese, chromium,molybdenum, nickel, boron and the like to a steel. Further, as theenlarging of the austenitic crystalline grain size was also known to bea method of improving the hardenability of the steel, it is possible tocarry out the hardening 0 process while heating the material at apossibly high temperature. However, this method is not recommendable,because the toughness of steel will be thereby largely impaired.

As to the effect of aluminum of influencing the hardenability of steelmaterial there are already some reports in a few documents. According tothe reports of these prior documents the following three kinds ofeffects of aluminum are referred to. The first of them is the effectwhich is said to relate to the size of crystalline grains and to be thestrongest amount other effects: that is, aluminum would make thecrystalline grains fine and would largely detriorate the hardenabilityof steel material. The second effect of aluminum is said to facilitatethe transformation of a steel and thereby to reduce the hardenability ofthe steel, though not certain, because aluminum oxide would liescattered in the form of fine particles in the steel, which wouldaccelerate the transformation as a nucleus of effecting thetransformation when carrying out the hardening process. Thirdly,aluminum is said to possess also a positive effect of improving thehardenability of steel material.

In summarizing these prior opinions on the effects of aluminum theprevailing opinion is that the effect of aluminum on the hardenabilityof steel material would be insignificant, or rather of a negativenature. According to the results of various experiments the hardeningmultiplying factor of aluminum is reported to be below 1.30 when causinga steel to contain 0.10 wt. percent. Therefore, no effect of aluminum ofimproving the hardenability of steel material has been expected.

In contrast with the prior opinions on the negative effect of aluminumon the hardenability of steel material the inventors have discovered anentirely unexpected novel fact that the hardenability of steel materialmay be remarkably improved by causing the steel to contain aluminum in arange of 0.06 to 0.15 wt. percent.

A further object of the present invention is to provide a low alloyedsteel having a high hardenability at a low price.

A still further object of the present invention is to provide a steelhigh in hardenability rendered by subsequently adding a specified amountof aluminum to the steel material of such a composition as being able tocause the ideal critical diameter, that is, the maximum diameter of aninfinitely long cylinder which in an ideal quench will just transform togiven specific mirco-structure, referring to the hardenability of steel,to reach more than 1.5 inches.

Other objects of the present invention will be made clear by makingreference to the following description and the attached drawing, inwhich;

FIG. 1 represents the effect of improving the hardenability of steelobtained by the addition of small amount of aluminum in correlation withthe ideal critical diameter D FIG. 2 is a diagram showing a relationbetween an amount of Al and the hardening multiplying factor of Al(MFAl).

In the following the features of the present invention will be explainedwith reference to a steel, in which the ideal critical diameter is morethan 1.5 inches. However, it is, of course, possible and effective toapply the idea of the present invention to a steel having the idealcritical diameter of less than 1.5 inches.

The said ideal critical diameter D may be given by multiplying the basichardenability (D which is again determined by the carbon content insteel and the crystalline grain size, by the hardenability multiplyingfactors of various alloying elements, as shown by the following formula:

XMF XMF XMF (l) in which C D (1nch)= -(lJO-ODQN) in which C=Carboncontent in steel (wt. percent) N=Grain size number of austeniticcrystalline grains MF =0.70-Si+1.00

MF =3.33-Mn+1.00 (in case of Mng1.2%) MF =5.10-(Mn-1.2)+5.00 (in case ofMn l.2%) MF ,=2.16-Cr+1.00

MF =3.00-Mo+1.00

MF =200.O-B+1.00 (in case of B 0.003%) In the above formula the amountsof the elements Si, Mn, Cr, Mo, V and B contained in the steel arerepresented by weight percent respectively.

The idea of the present invention is to remarkably improve thehardenability of a steel by adding an amount of aluminum more than 0.06wt. percent to the steel material, in which D shown in the above formulais more than 1.5 inches.

In case that a steel shows the ideal critical diameter D in the aboveFormula 1 of less than 1.0 inch, the steel corresponds to an ordinarycarbon steel, and the steel showing D more than 1.0 inch indicates a lowalloyed steel. The effect of the present invention of improving ahardenability of steel is particularly remarkable at the low alloyedsteel, particularly for use in machine construction, to which variousalloying elements are added so that the ideal critical diameter maybecome more than 1.5 inches. The steel of the present invention, inwhich D is more than 1.5 inches, is that which contains as alloyingelements 0.25 to 0.65 wt. percent, C, 0.005 to 0.30 wt. percent Si and0.20 to 2.00 wt. percent Mn 0.3 to 1.5 wt. percent Cr, 0.03 to 0.50 wt.percent Mo, 0.005 to 0.30 wt. percent V, 0.0005 to 0.003 wt. percent Band 0.06 to 0.15 wt. percent Al. These amounts of the above enumeratedcomponents to be contained in the steel of the present. invention havebeen calculated according to the above Formula 1 so that the idealcritical diameter D; of more than 1.5 inches may be obtained.

The present invention is to provide a steel high in hardenability byadding 0.06 to 0.15 wt. percent aluminum to the low alloyed steelmaterial of such a composition as being able to obtain the idealcritical diameter D of more than 1.5 inches.

The reasons of specifying the content of each alloying element are asfollows:

Among the basic elements, carbon, silicium and manganese, which areinevitably to be added, if the content of carbon is less than 0.25 wt.percent, the steel will lose the required strength and hardenability.But, if above 0.65 wt. percent, cracks are formed in the material duringthe hardening and sufficient toughness can not be obtained. It is verydifficult in the steel-making and consequently unpractical to reduce thecontent of silicon to less than 0.005 wt. percent. On the other hand,the content of silicon up to circa 0.30 wt. percent is in general anamount required for the steel-making, and in the present invention, ifthe silicon content exceeds 0.30 wt. percent, the hardening of steelbecomes difficult on account of an increase in macro streak flaws due todecarburization products and a rise in the transformation point.Manganese is also difiicult like silicium in the steel-making to reduceits content to below 0.20 wt. percent. But, if the content of manganeseis forcibly reduced to below 0.20 wt. percent, the hot-workability willbe deteriorated. But, if exceeding 2.00 wt. percent, the material willbecome brittle, though the strength may be improved as in the case ofsilicium.

In general, chromium is existent in an amount up to 0.03 wt. percent asone of impurities in a steel, but an addition of chromium in a range offrom 0.03 to 1.50 wt. percent is necessary for helping the effect ofaluminum of improving the hardenability of steel, and for imparting tothe steel the required strength.

Molybdenum has the same effect as chromium in helping the effect ofaluminum of improving the hardenability of steel, if it is added in arange from 0.03 to 0.50 wt. percent. Further, the addition of molybdenumin the above range is also required for preventing the temperbrittleness.

Vanadium is also added in a range of from 0.005 to 0.3 wt. percent alikechromium, molybdenum and nickel, as occasion calls. If the addition ofvanadium is less than 0.005 wt. percent, the effect of the addition cannot be materialized, but even if exceeding 0.3 wt. percent, the tempersoftening resistance can not be imparted and the effect of improving thestrength, toughness and hardenability proportional to the addition arenot achieved.

Boron may also be added to increase the effect of aluminum of improvingthe hardenability of steel, but in a range of from 0.0005 to 0.003 wt.percent. If below the above specified range, the effect of the additionof aluminum in improving the hardenability of steel can not be helpedand promoted thereby, but more than the specified range, the effectcorresponding to the increased addition can not be secured.

Thus, the steel according to the present invention is characterized bycontaining alloying elements such as chromium, molybdenum, vanadium,boron and in the range as specified above respectively, in addition tothe basic elements including carbon, silicium and manganese, so that theideal critical diameter D given by the Formula 1 may be more than 1.5inches and further containing aluminum subsequently added in a range offrom 0.06 to 0.15%.

In working the present invention, if the content of aluminum is lessthan 0.06 wt. percent, the sufficient hardening effect of aluminum isnot displayed, but if it is more than 0.06 wt. percent, thehardenability of steel is rapidly improved by the addition of aluminum,and the greater the ideal critical diameter D of the steel is, the morerapidly the hardenability is improved. As evidently seen from theattached drawings, FIGS. 1 and 2, the highest hardenability is shownwhen the content of aluminum lies in a range of from 0.06 to 0.10 wt.percent. For instance, in the case of the steel material, in which theideal critical diameter D is more than 3.0 inches,

the 50% martensite distance according to Iominy-test, that is, thedistance from the quenched end of the Iominytest piece to the point,where the 50% martensite structure has been formed, in the lengthdirection, reaches more than 40 mm., indicating a remarkable improvementof the hardenability of the steel material as compared with theconventional hardened steel material, in which the 50% martensitedistance according to Jominy-test shows only 11 mm. In general, thegreater the said 50% martensite distance according to Jominy-test is,the higher the hardenability of the steel material is. The above examplerelates to the advantages that the hardening by air cooling is feasibleand consequently complicated device and process required for thehardening by liquid cooling will be superfluous.

In FIG. 1 the dotted line shows the hardenability of the conventionalsteel material. For instance, the ideal critical diameter of the mediumcarbon steel, containing 0.45 wt. percent C, 0.75 wt. percent Mn and0.25 Wt. percent Si shows about 0.86 inch, supposing that the austeniticgrain size be 8. The hardenability of this steel material is judged fromthe attached drawing to be only 4 mm. in the 50% martensite distanceaccording to Jominy-test. In the steel material of this composition the50% martensite distance may be improved only to 11 mm., even if theideal critical diameter D is increased up to about 3.0 inches bysupplementally adding any alloying element for accelerating thehardenability, for instance, 1.2 wt. percent in the case of chromium,0.8 wt. percent in the case of molybdenum and more than 5 wt. percent inthe case of nickel. Further, it is to note that it is practicallydifficult to use such an alloying element in a sufficient amountnecessary for obtaining the sufiicient hardenability, because they arevery expensive.

Further, the relationship between an amount of Al measured in the caseof the alloyed steel manufactured according to the method of the presentinvention and MFAl (Multiplying Factor of Al) will be explained withreference to FIG. 2.

As above mentioned, in the steel of the present invention the content ofAl thereof is limited to the range of from 0.06 to 0.15 wt. percent,which just corresponds toughness due to coarsening of grains, anincrease in macto streak flaws and a promoted decarburization at asurface layer of steel. In this drawing also the MFA] values obtained bythe experiments of M. A. Grossman are shown for reference, as areindicated by MAG (1) and MAG (2), wherein MAG (1) is an MFAl lineobtained from FIG 23 in the article, Hardenability Calculated FromChemical Composition, by M. A. Grossman, page 249 in American Instituteof Mining and Metallurgical Engineering Transactions, vol. 150, 1942,pages 227255 and MAG (2) is an MFAl line obtained from FIG. 19 in thearticle, Elements of Hardenability, by M. A. Grossman (ASM 1952). As isclearly demonstrated by the comparison with these reference values, theMFAl value of the steel of the present invention is much superior tothese reference values. Further, it is to note that the presentinvention relates to a steel having D of more than 1.5 inches to whichAl is added in the range as above specified. On the other hand, thesteel shown in FIG. 23 in the article of M. A. Grossman, page 249, whichcontains maximum 0.08 wt. percent Al, has D of less than 1.5 inches,when calculated from the Formula 1. Such a striking hardening effect asis achieved by the present invention can never be expected for a steelhaving D of less than 1.5 inches.

The examples of the present invention are summarized in the followingtable, showing a comparison of the steels of the compositions asspecified by the present invention with the conventional steels in theirhardenabilities.

As evidently seen from the following table, although all alloyingelements excepting aluminum are substantially the same in both kinds ofsteels, the steels of the present invention show a marked superiority tothe conventional steels, as indicated by the ditferences in themartensite distances of them. Further, the conventional steels No. 4 andNo. 5 in the table are those, in which chromium is added in an increasedamount, while reducing the amount of manganese, in order to improve thehardenability of the steel, but the hardenabilities of them are still byfar inferior to those of the steels obtained by the addition of aluminumaccording to the present invention.

Chemical composition in wt. percent (balance Fe) Austenite J ominy-testSort Sanipl grain size D1 inch martensite N o. C Si Mn P S Cr Mo Al B VN 0. distance,

Steel of the present 1 0. 43 O. 38 1. 56 0. 020 0. 012 0. 32 0. 04 7. 53. 29 75. 0 invention. 2 0. 47 0. 25 1. 48 0. 017 0. 007 0. l8 0. 02 7.5 2. 71 24. 0 3 O. 46 0.27 1. 37 0.017 0.012 0.23 0. 16 7. 5 3. 20 25.04 0. 46 0. 26 1. 46 0. 012 0. 005 0. 21 0. 14 8. 0 3. 31 75. 0 5 0. 420. 23 1. 48 0. 012 0. 006 0. 18 0. 14 8.0 2. 87 46. 5 6 0. 39 0. 20 0.71 0.011 0. 005 0.96 0.04 8. 5 3. 25.0 7 0. 19 0. 35 1. 41 0. 013 0. 0060. 09 0. 02 8. 0 1. 67 16. 0 8 0. 44 0. 21 0. 83 0. 016 O. 010 0. 65 0.02 8. 0 2. 45 I4. 0 9 0. 48 O. 17 l. 44 0. 018 0. 005 0. 10 0. O2 7. 52. 58 23. 5 10 0. 43 0. 21 0. 90 0.013 0. 011 1.16 0.02 8 0 3. 39. 0 110.53 0. 2O 0. 86 0.010 0. 006 0.02 0.49 8.0 2. 56 35. 0 Conventionalsteel. l 0. 43 0.38 1. 56 0. 021 0.012 0.32 0. 04 7. 5 3. 10 12.0 2 O.47 0. 25 1. 46 0. 017 0. 007 0. 18 0. 02 7. 5 2. 58 8. 0 3 0. 47 0.25 1. 37 0. 016 0. 011 0. 23 0 15 7. 5 3. 20 9. 0 4 0. 45 0. 29 0.58 0.015 0.012 0. 67 7 0 1.82 6. 6 5 0. 40 0.28 0.75 0.016 0.013 1.15 7. 02.87 12.0

to the fact that MFAl shows such high values as above 1.4 which havenever been achieved heretofore, as is evidently shown by a line A and asphere enclosed with hatched lines a-b, 11-6 and cd in FIG. 2, when thecontent of Al lies within the above mentioned range. If the content ofAl is lower than 0.06 wt. percent, MFAl shows values less than 1.4, asis shown by a sphere enclosed with lines hg and g-f. Thus, a remarkableeffect of improving the hardenability by the addition of Al can not beexpected. On the other hand, if the content of Al exceeds 0.15 wt.percent, there occur various undesirable phenomena such as a reductionof Having thus described the invention, what is claimed is:

1. A steel of excellent mechanical properties and high hardenability,which has an ideal critical diameter greater than 1.5 inches, consistingof 7 8 1 0.0005 to 0.003 Weight percent B and balance being 3,216,82311/1965 Gulya 75-124 Fe and unavoidable impurities. 3,251,682 11/1966Wada 75124 References Cited HYLAND BIZOT, Primary Examiner.

UNITED STATES PATENTS 5 US Cl. XRI 3,110,586 11/1963 Gulya 75-124 X 7512g 3,155,495 11/1964 Nakarnura 75-124

